Abstract: Based on the finite-difference time-domain (FDTD) method and the transmission line (TL) equation, an efficient time-domain modeling algorithm is proposed, which can effectively solve the EM coupling modeling problem of microstrip line and realize the synchronous calculation of spatial EM fields and transient responses of microstrip line. Firstly, the per unit length distribution parameters of microstrip line are calculated based on the empirical formulas, and the TL equation suitable for EM coupling analysis of microstrip line is constructed. Then the FDTD method is used to simulate the excitation fields of microstrip line by combining non-uniform grid technique and automatic grid generation technique, which are introduced into the TL equation at each time step to obtain the equivalent distributed source terms. Finally, the central difference scheme of FDTD is used to discretize the TL equation to realize the iterative solutions of transient responses on microstrip line and its terminals. In order to verify the accuracy and efficiency of the time-domain modeling algorithm, the numerical simulation of EM coupling of microstrip line on PCB in free space and shielded cavity is carried out, and the calculation results of this proposed method are compared with those of traditional FDTD method in terms of calculation accuracy and time consumption.